Reaching the Sun and returning is probably the most difficult task one can do in this simulator.
To land on the Sun and return home is actually possible only in sandbox mode, because of the following problems:
- Solar gravity is too strong even for the most powerful engines which cannot lift the smallest required cargo (a 5-ton fuel tank and a probe).
- The corona will slightly slow down any rocket attempting to reach at a low heliocentric orbit.
- The amount of fuel needed is too large. Larger engines (by editing them in the files to make them larger) are required.
- There is no solid surface on the Sun. So, if you try to land without burning up, you will reach ground zero and continue to go down.
Still, in sandbox mode, with no gravity, no atmospheric drag and unlimited fuel, it is possible to reach ground zero and make it back home.
Mission[]
Using a huge amount of fuel, it is possible to send a rocket very close to the Sun, then return to Earth. The rocket will not actually land, but will get as close as possible to the Sun.
The main problem, as can be seen on the Delta-v chart, is that, reaching the Sun will require 30000 delta-v units. Compared, this is about 10 times more than the delta-v budget required to reach Mercury or Venus. The mission will also require to store the needed fuel for return, which in turn, will require far more fuel for getting to the Sun.
Rocket[]
The needed amount of fuel is far beyond what can be carried by a single rocket. It will require at least 10 launches (and probably even more) consisting mainly of fuel tanks, ion engines and sources of electricity. Then, all parts need to be assembled into a single, giant rocket. The rocket listed on top-right of this page is in a very low heliocentric orbit. It was 3 times larger when it left low Earth orbit and when it returned to Earth, it had only 7 tons of fuel left. It required 14 independent launches for bringing the parts, and many others for fuel supply.
The Ion Engine is probably the only way of propulsion for such a mission. Using rocket engines will require a rocket many times larger. The Solar Panel is not the best source of energy, because there is not enough room on the rocket for all panels needed, except for the last stages. The RTG is a better option.
When transferring fuel from one tank to another, the orbit will be slightly changed. Also, the many docking processes will change the orbit. In order to avoid getting too low, it is good to start assembling the rocket at a higher altitude.
Leaving Earth[]
Ion engines don't have the same thrust as other engines. At least at the beginning, the rocket will spiral away from Earth until it will escape its sphere of influence. Even with all the fuel stored onboard, it is very important to preserve as much as possible.
Lowering perihelion[]
A long, continuous engine burn will lower the perihelion down to the Sun. This can be started from Earth's distance, but it will be more efficient if started from further away (beyond Saturn or even Neptune) because of the Oberth effect.
If the rocket uses multiple gravity assists to get closer to the Sun (like from Mercury), the needed amount of fuel will be much higher, depending on the fuel needed for encounter maneuvers. The Parker Solar Probe used this method to go closer to the Sun, but because of the fuel and time needed for encounters, the probe will be at its lowest at around late 2020s or at the early 2030s.
Lowering aphelion[]
Below 1000 kilometers, the Sun's corona has a very small atmospheric drag. However, this effect is too little and it also rises the perihelion. The corona starts at 800,000 kilometers.
The corona has a very powerful burning effect and must be used with care. When reaching the atmosphere, the rocket will most likely melt.
Solar aerobraking is possible for versions older than 1.5.3, because those versions have no heating effects.
Closing in[]
A rocket can lower its orbit down to 300 kilometers. Beyond this value, it will enter the Sun's corona, which will make it melt. However, influences from the corona will alter the orbit. Because of this, it is better to fix a higher orbit (for example at 310 kilometers).
Once the rocket is at 310 kilometers, it is good to move fuel from a tank to another to dump empty tanks and to deploy a probe that will remain in orbit.
Returning to Earth[]
From low Sun orbit, a rocket needs to use tremendous amounts of fuel to return. It is good at first to use the engines continuously to spiral to a higher, safe altitude (for example 1000 kilometers to avoid perturbations from the corona).
From this altitude, in order to save some fuel, the rocket will fire its engines only around perihelion to gradually rise its aphelion. The process requires huge amounts of time and fuel. From time to time, the rocket will need to adjust rocket's inclination and to dump empty fuel containers.
Returning will require the highest amount of fuel.
Once a rocket has passed Mercury's orbit, it will be possible to use multiple Mercury and Venus gravity assists to get to Earth. Unfortunately, this will take much time, it will at first only rise the perihelion and will not save much fuel.
The delta-V budget needed to reach from low Sun orbit to Mercury's orbit is high, probably 20 times higher than the delta-V budget needed to reach from Mercury's orbit to the Earth. So, it makes more sense once a rocket crossed Mercury's orbit to continue burning until it crosses Earth's orbit.
Reaching Earth from the Sun is challenging. First of all, the rocket will approach on a highly elliptical orbit moving very fast compared to the Earth. Secondly, while coming from the Sun, the orbital path will not be very precise. The rocket will approach very fast and may not be slowed enough from Earth's atmosphere. It is, therefore, very important to conduct Trajectory Correction Maneuvers before reaching Earth. The rocket must go through Earth's atmosphere, but it is risky. The rocket will most likely burn up if you try to shoot up from the sun. Use some flybys to return to Earth not burning up.
Achievements[]
There are no official achievements for this type of mission.
Real missions[]
No records have been done for a manned mission to the Sun, although NASA's Genesis mission returned solar particles. Reports of NASA landing a an unmanned paperclip on the sun have been made.